Chetwynd
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chetwynd clear sky chart from 2005-11-06 to 2024-02-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
30%
23554
11%
81057
5%
23449
36%
23449
15%
55553

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
22%
1026
11%
4991
4%
1006
27%
1006
7%
3624
January11th to 20thNight
21%
924
17%
4802
5%
911
29%
911
7%
3285
January21st to endNight
19%
927
8%
5180
5%
902
30%
902
6%
3510
February1st to 10thNight
30%
763
11%
2637
9%
763
34%
763
12%
1899
February11th to 20thNight
26%
701
12%
2315
7%
700
33%
700
13%
1690
February21st to endNight
29%
592
11%
1924
7%
592
36%
592
13%
1401
March1st to 10thNight
30%
720
15%
2176
9%
720
36%
720
17%
1552
March11th to 20thNight
30%
720
14%
2020
7%
720
35%
720
18%
1440
March21st to endNight
35%
792
14%
2056
6%
792
39%
792
20%
1502
April1st to 10thNight
38%
720
12%
1800
4%
720
43%
720
24%
1263
April11th to 20thNight
37%
497
13%
1565
9%
497
43%
497
17%
1081
April21st to endNight
32%
360
10%
1440
10%
360
41%
360
14%
968
May1st to 10thNight
37%
360
6%
1440
5%
360
39%
360
18%
904
May11th to 20thNight
46%
354
1%
1174
0%
354
46%
354
24%
711
May21st to endNight
43%
389
0%
1164
0%
388
44%
388
19%
813
June1st to 10thNight
37%
360
0%
1080
0%
360
37%
360
19%
750
June11th to 20thNight
37%
360
0%
1080
0%
360
37%
360
16%
810
June21st to endNight
47%
360
0%
1080
0%
360
47%
360
26%
683
July1st to 10thNight
43%
350
0%
1050
0%
350
43%
350
23%
703
July11th to 20thNight
40%
348
0%
1044
0%
348
40%
348
24%
693
July21st to endNight
49%
394
0%
1275
0%
372
51%
372
28%
719
August1st to 10thNight
53%
358
5%
1335
4%
356
55%
356
23%
791
August11th to 20thNight
45%
356
13%
1424
13%
356
55%
356
24%
782
August21st to endNight
43%
541
14%
1729
11%
541
50%
541
24%
1093
September1st to 10thNight
36%
720
11%
1800
6%
720
39%
720
27%
1239
September11th to 20thNight
41%
720
18%
1800
8%
720
45%
720
29%
1199
September21st to endNight
30%
720
16%
2046
7%
720
35%
720
18%
1424
October1st to 10thNight
31%
720
11%
2160
5%
720
38%
720
18%
1492
October11th to 20thNight
26%
720
14%
2231
7%
720
36%
720
17%
1532
October21st to endNight
22%
792
12%
2713
5%
792
33%
792
13%
1889
November1st to 10thNight
25%
740
15%
2590
12%
740
33%
740
13%
1849
November11th to 20thNight
19%
905
12%
2805
4%
905
28%
905
11%
2051
November21st to endNight
19%
1031
10%
3021
3%
1031
26%
1031
14%
2205
December1st to 10thNight
21%
1040
14%
3040
4%
1040
30%
1040
15%
2183
December11th to 20thNight
24%
1039
12%
3689
3%
1039
31%
1039
11%
2695
December21st to endNight
26%
1135
9%
5381
5%
1114
34%
1114
10%
3128

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
9%10%48%7%46%6%6%8%32%
77211
Night
8%11%5%8%7%46%6%7%8%32%
81591
TransparencyDay
48%7%17%20%7%1
76899
Night
48%8%16%20%7%1
81484
SeeingDay
18%16%29%23%11%3
15530
Night
16%16%28%26%13%2
23606

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
311%6%13%10%6%9%7%6%7%21%
1899
11th to 20thDay
5%13%410%7%5%10%6%11%6%22%
2071
21st to endDay
6%11%48%7%6%6%5%7%9%31%
2442
February1st to 10thDay
6%10%5%9%9%6%9%5%6%6%30%
1416
11th to 20thDay
314%7%10%7%5%6%6%6%7%29%
1530
21st to endDay
310%7%12%10%48%6%6%6%27%
1332
March1st to 10thDay
412%6%12%10%5%6%5%5%7%29%
1784
11th to 20thDay
6%14%6%10%6%36%5%6%8%31%
1935
21st to endDay
5%16%7%10%7%46%6%6%8%26%
2300
April1st to 10thDay
5%11%6%11%9%5%7%5%7%7%28%
2160
11th to 20thDay
9%14%6%10%8%46%5%6%6%26%
2341
21st to endDay
12%12%5%8%6%36%5%7%6%30%
2520
May1st to 10thDay
10%10%5%8%7%46%6%6%8%30%
2520
11th to 20thDay
12%8%37%6%46%5%6%7%35%
2719
21st to endDay
17%7%47%7%46%5%7%8%27%
3111
June1st to 10thDay
12%7%46%8%46%6%7%9%30%
2880
11th to 20thDay
16%9%49%8%46%6%6%7%26%
2880
21st to endDay
9%5%36%6%5%7%6%7%8%37%
2880
July1st to 10thDay
12%5%36%6%5%6%7%6%8%36%
2880
11th to 20thDay
14%6%36%6%37%6%7%8%34%
2880
21st to endDay
9%5%36%6%36%6%7%8%40%
2965
August1st to 10thDay
10%6%26%7%46%5%7%9%37%
2597
11th to 20thDay
9%425%5%45%6%7%9%44%
2475
21st to endDay
12%6%36%7%46%7%7%7%36%
2623
September1st to 10thDay
13%6%36%5%35%5%6%7%41%
2160
11th to 20thDay
13%8%37%5%35%5%6%8%37%
2160
21st to endDay
10%9%47%7%47%6%7%9%32%
1911
October1st to 10thDay
12%13%36%6%45%5%5%7%34%
1800
11th to 20thDay
10%11%5%7%7%45%5%5%7%32%
1728
21st to endDay
11%14%48%6%35%5%5%6%32%
1642
November1st to 10thDay
12%14%6%9%6%35%46%6%30%
1488
11th to 20thDay
6%15%6%9%9%45%45%7%29%
1354
21st to endDay
6%13%5%9%10%35%5%6%8%30%
1156
December1st to 10thDay
8%12%6%9%8%5%5%46%5%32%
1140
11th to 20thDay
6%14%5%10%7%5%7%7%6%8%26%
1442
21st to endDay
48%46%6%46%7%8%9%38%
2090
January1st to 10thNight
414%7%14%9%5%6%5%6%7%24%
5046
11th to 20thNight
5%10%5%10%10%5%7%8%12%8%20%
4855
21st to endNight
7%13%5%8%7%5%6%6%8%9%26%
5214
February1st to 10thNight
7%14%5%9%9%5%6%5%5%7%27%
2637
11th to 20thNight
5%13%6%10%9%46%7%6%8%25%
2313
21st to endNight
310%8%10%9%5%7%6%6%7%28%
1924
March1st to 10thNight
313%6%10%8%5%6%5%7%8%30%
2176
11th to 20thNight
5%11%7%10%7%35%5%7%9%31%
2020
21st to endNight
7%13%6%9%7%35%5%6%7%31%
2056
April1st to 10thNight
49%48%7%5%6%6%6%10%34%
1800
11th to 20thNight
8%13%5%7%7%36%5%5%8%33%
1565
21st to endNight
11%13%37%5%45%5%6%8%31%
1440
May1st to 10thNight
11%9%5%7%6%4346%9%36%
1440
11th to 20thNight
14%8%24435%5%7%9%40%
1174
21st to endNight
16%7%45%6%46%7%7%8%30%
1164
June1st to 10thNight
12%9%48%7%45%5%5%6%35%
1080
11th to 20thNight
15%9%49%7%5%7%5%7%9%25%
1080
21st to endNight
10%5%38%5%446%8%11%36%
1080
July1st to 10thNight
11%6%36%7%46%5%7%8%37%
1080
11th to 20thNight
10%7%46%6%36%5%9%10%34%
1080
21st to endNight
7%3246%35%6%7%11%45%
1350
August1st to 10thNight
10%425%6%35%6%7%8%43%
1342
11th to 20thNight
7%4246%35%5%9%8%47%
1424
21st to endNight
10%5%35%5%45%5%7%9%40%
1729
September1st to 10thNight
13%7%26%6%35%46%7%42%
1800
11th to 20thNight
11%7%25%5%35%5%6%8%43%
1800
21st to endNight
11%10%36%6%35%5%6%8%37%
2046
October1st to 10thNight
12%11%37%5%35%6%7%8%34%
2160
11th to 20thNight
10%11%47%6%45%6%5%9%34%
2231
21st to endNight
11%12%5%6%7%35%6%6%7%32%
2713
November1st to 10thNight
10%14%5%7%7%46%45%7%30%
2590
11th to 20thNight
7%14%5%11%7%45%45%7%31%
2805
21st to endNight
6%12%5%11%9%5%5%5%6%7%29%
3021
December1st to 10thNight
6%10%6%9%9%45%5%5%8%33%
3040
11th to 20thNight
7%13%5%10%6%47%6%6%7%30%
3750
21st to endNight
38%47%7%5%6%7%8%8%37%
5566

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
58%9%16%14%4
1899
11th to 20thDay
56%7%14%9%12%1
2092
21st to endDay
48%7%25%16%5%
2442
February1st to 10thDay
53%7%14%13%11%1
1416
11th to 20thDay
52%10%15%18%5%
1539
21st to endDay
56%7%14%16%6%1
1332
March1st to 10thDay
54%6%13%19%9%1
1784
11th to 20thDay
50%6%18%19%7%1
1935
21st to endDay
55%7%17%15%6%
2300
April1st to 10thDay
52%7%13%19%8%1
2160
11th to 20thDay
57%6%11%18%7%1
2341
21st to endDay
52%6%14%20%7%1
2520
May1st to 10thDay
49%8%14%21%7%1
2520
11th to 20thDay
46%6%13%21%13%1
2723
21st to endDay
52%7%13%20%7%1
3112
June1st to 10thDay
47%8%14%21%9%1
2880
11th to 20thDay
54%7%13%18%7%
2880
21st to endDay
41%9%17%23%10%1
2880
July1st to 10thDay
41%8%18%22%9%1
2801
11th to 20thDay
46%8%21%22%31
2793
21st to endDay
38%7%23%27%5%
2800
August1st to 10thDay
42%8%23%22%5%
2579
11th to 20thDay
35%8%23%27%7%
2475
21st to endDay
43%9%20%23%6%
2623
September1st to 10thDay
40%7%24%23%5%1
2160
11th to 20thDay
44%7%18%24%7%
2160
21st to endDay
46%9%20%19%6%1
1911
October1st to 10thDay
49%7%18%21%5%
1800
11th to 20thDay
49%7%17%24%4
1728
21st to endDay
50%6%19%19%6%
1642
November1st to 10thDay
53%8%13%21%5%1
1488
11th to 20thDay
55%7%14%17%6%1
1354
21st to endDay
50%8%17%17%7%1
1156
December1st to 10thDay
53%6%13%18%9%1
1140
11th to 20thDay
54%7%17%17%5%
1444
21st to endDay
38%8%21%23%10%1
2090
January1st to 10thNight
56%8%13%16%6%1
5046
11th to 20thNight
52%8%12%13%14%1
4896
21st to endNight
52%8%19%15%5%
5214
February1st to 10thNight
55%5%14%15%10%
2637
11th to 20thNight
53%9%17%17%5%
2315
21st to endNight
53%7%13%18%7%1
1924
March1st to 10thNight
50%7%15%20%7%1
2176
11th to 20thNight
47%8%15%23%6%1
2021
21st to endNight
50%6%16%20%8%
2056
April1st to 10thNight
42%9%17%23%9%1
1800
11th to 20thNight
49%6%12%25%8%
1565
21st to endNight
50%6%11%24%7%2
1440
May1st to 10thNight
44%6%15%25%9%
1440
11th to 20thNight
39%6%15%26%14%1
1174
21st to endNight
48%7%15%19%10%1
1164
June1st to 10thNight
48%6%11%24%10%
1080
11th to 20thNight
55%7%11%20%7%1
1080
21st to endNight
38%7%20%25%10%1
1080
July1st to 10thNight
43%7%15%26%8%
1050
11th to 20thNight
42%7%24%21%5%
1044
21st to endNight
30%9%26%30%5%
1275
August1st to 10thNight
36%9%21%26%9%
1335
11th to 20thNight
31%8%22%29%9%1
1424
21st to endNight
39%8%21%26%6%
1729
September1st to 10thNight
42%6%19%26%6%1
1800
11th to 20thNight
37%8%21%29%5%
1800
21st to endNight
43%10%17%22%7%1
2046
October1st to 10thNight
46%7%21%20%6%1
2160
11th to 20thNight
46%8%19%22%5%
2231
21st to endNight
49%8%16%20%7%
2713
November1st to 10thNight
52%8%14%20%5%1
2590
11th to 20thNight
53%7%14%19%6%
2805
21st to endNight
52%7%16%18%5%1
3021
December1st to 10thNight
49%7%16%18%9%1
3040
11th to 20thNight
52%9%15%18%6%
3747
21st to endNight
41%8%21%21%8%2
5566

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
14%16%32%27%7%4
180
11th to 20thDay
17%18%27%24%12%3
180
21st to endDay
22%17%30%21%9%1
198
February1st to 10thDay
18%20%30%20%10%1
168
11th to 20thDay
17%20%36%18%9%
290
21st to endDay
14%19%36%20%9%1
280
March1st to 10thDay
15%19%29%22%13%2
414
11th to 20thDay
14%21%34%22%8%2
420
21st to endDay
21%16%29%21%13%1
462
April1st to 10thDay
20%16%30%23%8%3
420
11th to 20thDay
22%16%31%18%11%2
412
21st to endDay
22%20%31%18%8%2
420
May1st to 10thDay
19%21%30%21%8%1
423
11th to 20thDay
17%13%26%27%13%4
688
21st to endDay
19%14%24%25%15%4
874
June1st to 10thDay
16%17%30%21%13%1
810
11th to 20thDay
21%15%26%24%11%2
810
21st to endDay
12%14%26%25%17%6%
810
July1st to 10thDay
15%12%25%29%16%2
796
11th to 20thDay
16%11%33%21%16%3
792
21st to endDay
12%13%34%27%9%5%
711
August1st to 10thDay
14%11%25%28%18%4
537
11th to 20thDay
14%20%33%17%11%4
437
21st to endDay
19%20%29%21%8%3
495
September1st to 10thDay
21%16%31%18%9%4
450
11th to 20thDay
21%16%28%23%9%4
450
21st to endDay
16%27%32%18%6%
450
October1st to 10thDay
28%16%27%20%6%2
450
11th to 20thDay
25%17%29%21%7%1
378
21st to endDay
23%14%31%20%9%2
227
November1st to 10thDay
22%26%26%17%7%2
180
11th to 20thDay
25%21%33%15%5%1
180
21st to endDay
24%21%27%18%8%1
180
December1st to 10thDay
15%22%27%27%7%2
180
11th to 20thDay
17%22%24%21%13%2
180
21st to endDay
16%15%24%28%13%4
198
January1st to 10thNight
13%18%33%26%9%2
1040
11th to 20thNight
16%20%30%23%9%1
941
21st to endNight
19%23%29%22%6%1
935
February1st to 10thNight
19%17%22%25%15%2
763
11th to 20thNight
14%17%29%27%12%1
704
21st to endNight
10%15%31%27%16%1
592
March1st to 10thNight
11%17%30%26%13%2
720
11th to 20thNight
11%18%31%28%12%1
720
21st to endNight
14%13%24%25%21%2
792
April1st to 10thNight
10%14%28%29%18%2
720
11th to 20thNight
16%14%23%27%18%2
497
21st to endNight
19%13%26%25%16%1
360
May1st to 10thNight
16%12%26%27%16%3
360
11th to 20thNight
17%8%22%25%27%2
354
21st to endNight
18%10%19%28%24%2
389
June1st to 10thNight
16%9%28%28%18%2
360
11th to 20thNight
21%7%26%25%19%3
360
21st to endNight
13%8%25%30%21%3
360
July1st to 10thNight
14%6%27%35%16%1
350
11th to 20thNight
14%7%28%28%21%1
348
21st to endNight
8%10%26%32%20%3
394
August1st to 10thNight
13%5%20%28%28%6%
358
11th to 20thNight
8%13%27%29%21%2
356
21st to endNight
13%7%27%33%17%3
541
September1st to 10thNight
16%10%29%28%15%1
720
11th to 20thNight
16%13%23%26%20%3
720
21st to endNight
19%21%26%25%9%1
720
October1st to 10thNight
22%15%24%28%10%1
720
11th to 20thNight
18%17%30%26%9%1
720
21st to endNight
24%18%28%23%7%
792
November1st to 10thNight
21%18%27%24%9%2
740
11th to 20thNight
17%23%32%23%5%
905
21st to endNight
19%21%30%23%6%
1031
December1st to 10thNight
15%20%35%23%6%1
1040
11th to 20thNight
17%19%30%24%9%1
1040
21st to endNight
14%18%28%28%10%2
1144
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